1. Technical Field
The present disclosure relates to a pressure sensor having a high full-scale value with a package thereof and, in particular, in the following description explicit reference will be made, without implying any loss in generality, to use of the pressure sensor in an electromechanical braking system of the BBW (Brake By Wire) type.
2. Description of the Related Art
As is known, disc-braking systems of a traditional type for vehicles have a disc fixed to a respective wheel of the vehicle, a caliper associated with the disc, and a hydraulic control circuit. The caliper houses pads (generally two in number) made of friction material, and one or more pistons connected to the hydraulic control circuit. Due to pressure exerted by a user on the brake pedal, a pump in the hydraulic control circuit pressurizes a fluid contained in the circuit. Consequently, the pistons, which are equipped with appropriate seal elements, leave their respective seats and press the pads against the surface of the disc, thus exerting a braking action on the wheel.
Recently, systems have been proposed referred to as “Drive by Wire”, which envisage electronic control of the main functions of a vehicle, for example the steering system, the clutch, and the braking system. In particular, braking systems with electronic control have been proposed, which envisage replacing the hydraulic calipers with actuators of an electromechanical type. In detail, appropriate sensors detect actuation of the brake pedal, and generate corresponding electrical signals, which are received and interpreted by an electronic control unit. The electronic control unit then controls intervention of the electromechanical actuators (for example, pistons actuated by an electric motor), which exert the braking action on the corresponding brake discs through the pads. The electronic control unit moreover receives from sensors associated with the braking system information on the braking action exerted by the electromechanical actuators so as to provide an appropriate closed-loop feedback control, for example, via a proportional-integral-derivative (PID) controller. In particular, the electronic control unit receives information on the pressure exerted by each actuator on the respective disc brake.
In order to measure the pressure, pressure sensors with a high full-scale value are necessary. In fact, the force with which the pads are pressed against the disc can assume values from 0 N up to a maximum value in the range of 15,000 N-35,000 N. The piston acting on the pads has a cross section in the region of 2 cm2, so that the pressure sensors must operate up to full-scale values in the region of 1700 kg/cm2 or more. Furthermore, it is advantageous to perform a pressure measurement with a dual measurement scale for measuring both low-pressure values with a first precision and high-pressures values with a second precision that is lower than the first precision.
The patent application PCT/IT05/00435, filed on Jul. 22, 2005, in the name of the present applicant, describes an integrated pressure-sensor element with a high full-scale value made with the monolithic-silicon technology.
More particularly, in detail (FIG. 1), the pressure-sensor element, designated by 1, comprises a monolithic body 2 made of semiconductor material, and has a first main surface 2a and a second main surface 2b opposite to one another. A stress resulting from a pressure P to be determined acts on both surfaces. The monolithic body 2 is solid and compact, is preferably made of monocrystalline silicon of an N type with (100) orientation of the crystallographic plane, and has a square cross section, for example, 800 μm×800 μm. The monolithic body 2 moreover has a thickness W (understood as distance between a first main outer surface thereof and a second main outer surface thereof that are opposite to one another) that is substantially uniform, for example, of 400 μm, and further comprises a bulk region 3 and piezoresistive detection elements 4. The piezoresistive detection elements 4 comprise doped regions, obtained, for example, by diffusion, and are made within a portion of the bulk region 3 arranged in the proximity of the first main surface 2a. In particular, the bulk region 3 is a solid and compact region (without empty spaces), having a respective thickness substantially equal to the thickness W. Furthermore, a passivation layer 5 (made, for example, of silicon oxide) coats the main outer surface of the monolithic body 2 adjacent to the piezoresistive detection elements 4.
The general operation of the pressure-sensor element 1 is based upon the so-called piezoresistive effect, whereby a stress applied to a piezoresistive material causes a variation of resistance thereof. In the case of semiconductor materials, such as silicon, the applied stress determines a deformation of the crystal lattice and hence an alteration of the mobility of the majority charge carriers. Deriving therefrom is a variation in the resistivity of piezoresistive elements formed in the semiconductor material. In particular, the pressure P determines a stress in a direction normal to the first main surface 2a, which causes a variation in the resistance of the piezoresistive detection elements 4. Said variation is detected by an appropriate Wheatstone-bridge measuring circuit (not illustrated) in order to determine the value of the pressure P.
The patent application PCT/IT05/00431 filed on Jul. 22, 2005, in the name of the present applicant, further describes an integrated pressure-sensor element with a high full-scale value and a double measurement scale, made with the monolithic-silicon technology.
More particularly, in detail (see FIG. 2), the pressure-sensor element, designated by 10, comprises a monolithic body 12 of semiconductor material, preferably monocrystalline silicon, and has a first main surface 12a and a second main surface 12b opposite to one another. A stress resulting from the pressure P, the value of which is to be determined, acts on both surfaces. The monolithic body 12 is preferably made of monocrystalline silicon of an N type with (100) orientation of the crystallographic plane, and has a square cross section, for example, 15 mm×15 mm, and a substantially uniform thickness W, for example, of 375 μm. The monolithic body 12 further comprises a bulk region 13 and a cavity 14, buried in the monolithic body 12.
For example, the cavity 14 can be formed with the manufacturing process described in patent application EP 04 425 197.3 filed in the name of the present applicant on Mar. 19, 2004.
In summary, said process initially envisages depositing on the monolithic body 12 a resist layer, which is then defined so as to form a mask. The mask has an area of an approximately square shape and comprises a plurality of hexagonal mask portions that define a honeycomb lattice. Then, through the mask, an anisotropic chemical etching of the monolithic body 12 is performed, following upon which trenches are formed, which communicate with one another and delimit a plurality of silicon pillars. In practice, the trenches form an open region of a complex shape in which the pillars extend. Next, the mask is removed and an epitaxial growth is performed in a deoxidizing environment. Consequently, an epitaxial layer grows above the pillars and closes the open region at the top. A step of thermal annealing is then carried out, which causes a migration of the silicon atoms, which tend to move into position of lower energy. Consequently, and also due to the small distance between the pillars, the silicon atoms migrate completely from the portions of the pillars within the open region, and the cavity 14 is consequently formed. A thin silicon layer remains above the cavity 14, constituted in part by epitaxially grown silicon atoms and in part by migrated silicon atoms, which forms a membrane 15.
The cavity 14 has, for example, a square cross section, 300 μm×300 μm, and a thickness of approximately 1 μm (in particular, the size of the cavity 14 is quite negligible as compared to the size of the monolithic body 12). The membrane 15, for example having the thickness of 0.5 μm, is laterally surrounded by the bulk region 13, is flexible and deforms as a function of the pressure P acting on the monolithic body 12. First piezoresistive detection elements 16 are provided within the membrane 15, and comprise doped regions, obtained, for example, by diffusion, the resistance of which varies as a function of the deformation of the membrane 15. The first piezoresistive detection elements 16 are connected in a first Wheatstone-bridge sensing circuit (not illustrated).
In a surface portion of the bulk region 13, in a position separated and distinct from the membrane 15, second piezoresistive detection elements 18 are provided, which also comprise doped regions, for example, obtained by diffusion, which are separated from the membrane 15 by a distance, for example not less than 50 μm, such as not to feel the effect of the stresses acting on the membrane 15 and of its deformation. In particular, the second piezoresistive detection elements 18 are integrated in a portion of the bulk region 13 that is solid and compact (without empty spaces), having a respective thickness substantially equal to the thickness W. Furthermore, the second piezoresistive detection elements 18 are connected in a second Wheatstone-bridge sensing circuit (not illustrated), distinct from the first sensing circuit. In particular, the second piezoresistive detection elements 18 are not electrically connected to the first piezoresistive detection elements 16. A focusing region 19a, of silicon oxide, is arranged on the monolithic body 12, at the membrane 15, so as to focus the pressure P on the membrane 15, forcing it to deform. Furthermore, a passivation layer 19b, which also is, for example, of silicon oxide, is arranged on the focusing region 19a. 
Operation of the pressure-sensor element 10 is again based upon the piezoresistive effect of monocrystalline silicon. In particular, the pressure P to be measured determines a stress in a direction normal to the first main surface 12a, which causes a deformation of the membrane 15. Said deformation induces a longitudinal and transverse mechanical stress in the first piezoresistive detection elements 16, which undergo a variation of resistance, which is detected by the first Wheatstone-bridge sensing circuit. The pressure P moreover determines on each of the second piezoresistive detection elements 18 a transverse stress that determines a variation of their resistance, which can be detected by the second Wheatstone-bridge sensing circuit. In detail, for low values of the pressure P, the deformation of the second piezoresistive detection elements 18 is practically negligible. Instead, the membrane 15 is induced to deform by the focusing region 19a, causing a corresponding deformation of the first piezoresistive detection elements 16. As the pressure P increases, the deformation of the membrane 15 increases, until the membrane 15 contacts the bottom of the underlying cavity 14, thus saturating the pressure value supplied at output (in so far as no further deformations are possible). In particular, said saturation occurs for values of the pressure P in the range of around 10-15 kg/cm2. At this point, a further increase in the pressure P starts affecting the entire outer surface of the monolithic body 12, causing a non-negligible variation in the resistance of the second piezoresistive detection elements 18 from which the value of the pressure P is obtained. The pressure-sensor element 10 thus has two independent and complementary measurement scales: a first measurement scale, which is valid for low values of the pressure P and has a full scale of around 10-15 kg/cm2 (due to the action of the membrane 15 and of the first piezoresistive detection elements 16), and a second measurement scale, which is valid for high values of the pressure P and has a full scale of around 2000 kg/cm2 (due to the action of the second piezoresistive detection elements 18). The first measurement scale is more precise than the second, given that the membrane 15 is sensitive to even minimal pressure variations.
In both of the described pressure sensors, given the crystalline nature of silicon and the high values of pressure involved, the presence of a first buffer layer and of a second buffer layer is necessary, said buffer layers being made of a solid elastic material, for example steel, and being arranged, respectively, on the first main surface 2a, 12a, and underneath the second main surface 2b, 12b of the pressure-sensor element 1, 10. The buffer layers have the function of uniformly distributing the applied pressure over all the useful surface, preventing any “focusing” that might cause cracks along the crystallographic axes. In FIG. 1 and FIG. 2, the first and second buffer layers are designated by 20a and 20b. 
In detail, the first buffer layer 20a and the second buffer layer 20b require a high precision in the machining stage to guarantee absolute planarity of the respective faces in contact with the pressure-sensor element. In fact, as is schematically shown in FIG. 3, which refers by way of example to the pressure-sensor element 1 of FIG. 1, the presence of projecting asperities 21 on the face of the first buffer layer 20a and/or of the second buffer layer 20b set directly in contact with the pressure-sensor element 1 determines focusing of the applied pressure P in given points of the monolithic body 2, with the possibility of the silicon mechanical resistance being locally exceeded and of cracks 23 forming in the monolithic silicon body 2.